With regular forces taking priority for equipment, the Home Guard volunteers were at first poorly armed with their own weapons or obsolete firearms. The original idea was that the guard would act as a secondary defensive force behind the Army, guarding roads, canals and airfields.

But they were soon converted to a more aggressive role to deal with a potential German invasion, Dr Johnston said.

The German forces planned to push north and encircle London. The German plan estimated that if they advanced as far as Northampton, the UK would surrender.

If the Home Guard were to stand and fight well-trained and well-equipped German troops who had pioneered and mastered Blitzkrieg warfare, they would suffer enormous casualties for little gain it was decided.

So the guard was reimagined as a guerrilla force that would limit the German advance.

Dr Johnston said: “They would be a secret army, sabotaging and attacking the enemy from the rear, slowing them down to buy time for the regular Army to regroup further inland and re-establish defensive positions.”

The Home Guard drilled twice a week, took part in regular camps, and was trained by the regular Army. Within a short space of time it was probably the equivalent of today’s Army Reserve, he said.

Life in the Home Guard was also dangerous with 1,206 members being killed in the war.

Though the invasion never came, the Home Guard remained active in British defence and manned anti-aircraft guns. The Home Guard was stood down in December 1944.

Dr Johnston said: “Had the invasion come in the summer of 1940, the Home Guard would undoubtedly have fought with great commitment but probably been able to accomplish little. But the longer the delay in the planned German invasion went on, the stronger the Home Guard, and British forces overall, became, and better able to resist any prospective assault. They were a vital part of Britain’s defences throughout the war.”